What kind of art? Realistic, superhero, color or b/w, line art or shaded art, what? Also, if you want a good artist you have to pay upfront, or at least promise them (get it in contract) at least half of the profit sharing from any merchandise, subscriptions, etc. No professional is going to ink a long-running comic series "for fun" or only the possibility of future earnings. I'm an amateur and I don't even think I'd do it, I need all my free time for my own projects. Sorry if I'm being harsh but drawing art for others is a job just like a "real" job.